CLASSIFICATION UV THE THERIOUuNTlA. 77 



stage ill the advance in structure of the basicranial region, 

 already discussed in F. Z. S. ll;)14, p. 1027, figs. 3 &, 4; it is in 

 this region more advanced than any other known Endothlodon- 

 zone Gorgonopsid. It retains as primitive features the very large 

 quadrate, large postfrontal and frontal, and a sloping occiput. 



Thus the Eiidothiodon-zowB Gorgonopsids show definite ad- 

 vances over the Pelycosaurs in the direction of Dlademodon. 

 Eacli form is advanced in certain features Avhilst retaining a 

 more primitive structure in others, so that an imaginary animal, 

 built up by throwing together the most advanced features found 

 ill all the actual animals, would be far more advanced than any 

 one is on the average ; although in no point would it be more 

 advanced than a known form. In fact, the evidence existing 

 here, small though it is, suggests that there is a limit to the 

 total amount of advance possible to the members of a group in a 

 given time, and that these changes 'ma,y be distributed either 

 over the whole animal or concentrated on a definite region, which 

 will then present a structure of much more advanced type than 

 is found in allied contemporaneous forms. A somewhat similar 

 conclusion seems to have been reached by W. D. Matthew from 

 the study of the more abvmdant material of fossil mammals. 



Discussion of the Gorgonopsids of the Cistecephcd as-zone is 

 rendered difficult by two factors — the incomplete descriptions 

 and insufficient figures of many of the perfect skulls in S. Africa 

 and New York, and the fact that the (Jistecephalus-zone is a 

 long one and that we do not know the relative ages of the 

 Gorgonopsids from it. It will appear from the evidence to be 

 brought forward in this paper that the forms from Dunedin and 

 Nieuweveld localities are early, those from New Bethesda and 

 the Kagaberg which are associated with Dlcynodon tiyriceps 

 considei-ably later in time. There is, however, no stratigraphical 

 evidence that this is so. 



From the Cistecephalns-zone Broom and llaughton have 

 described several forms as species of 8c>/mnoynathus — »S'. tiyricejps 

 B. ife H., S. parvas Br., ;S'. minor Br., S. angastlceps Br., S.serra- 

 tideus Hau., are all from the Nieuweveld. These forms may 

 very possibly be congeneric ; they agree with ScymnognatlitijS in 

 having i. 5, c. 1, m. 4-5, but quite certainly do not belong to 

 that genus. They differ fi-om Scumuoyiiathus ivhaitsl in the 

 following characters : — 



The snout is very much deeper, its anterior end instead of 

 being rounded is vertical {cf. Broom,P. Z.S. 1913, p. 225, pi. 36), 

 the external nostril is much larger, the septoniaxillary foramen 

 smaller. The anterior end of the nasal does not fully overhang 

 the nosti'il. The top of the snout may be ridged, and the square 

 section with a preorbital depression overhung b}" the prefrontal 

 is entirely lost (cf. S. serratidens, Ann. South Afr. Mus. vol. xii. 

 p. 89, pi. xiii.). The snout is much shorter and the prefrontal 

 in consequence smaller. 



